I know plenty of people who actively help other players get through outdated content. The entire game is set up in such a way as to ensure that there's a constant flow of players tackling new and old content alike.
Would additional incentives be a good idea? Sure - but they're not necessarily a necessity and the sort of player who needs an incentive to help out other players isn't likely to go out of their way to aid others in the first place.
Would additional incentives be a good idea? Sure - but they're not necessarily a necessity and the sort of player who needs an incentive to help out other players isn't likely to go out of their way to aid others in the first place.